This invention relates generally to ultrasensitive sensors, including sensors that may be used individually or that may be used in an array in artificial olfactory systems, and more particularly to the attachment of sensing materials to substrates, most particularly porphyrin sensing materials to aerogel substrates, in ultrasensitive sensors.
The human olfactory system has about 100 million olfactory cells and each cell has about 10 cilia resulting in about 1 billion sensing elements. The olfactory system of canines has orders of magnitude more sensing elements. It is this enormous number of sensing elements that gives the ultrasensitivity to biological olfactory systems. While the number of olfactory sensors is very large, the number of different olfactory sensors is much smaller, about 1000 in a human. Identification of an odor is through pattern recognition and neural processing.
Artificial olfactory systems attempt to model the biological olfactory system. Arrays of nonspecific chemical sensors are used with signal processing to identify odorants through pattern recognition. Artificial olfactory systems are presently limited by the sensitivity of the sensors and the processing techniques.
U.S. Pat. No. 6,598,459 to Chi Yung Fu describes an approach to an ultrasensitive sensor element and array for an artificial olfactory system. Ultrasensitivity is obtained by producing a very large surface area on the sensor to mimic the very large number of sensing elements in the biological system. Also sophisticated fuzzy logic and neural network processing are used to identify the detected patterns. The sensor is formed with a substrate of a very high surface area material (a “surface area increasing material”) on a conventional sensor body, typically an acoustic device or resonator such as a quartz crystal microbalance (QCM), a surface acoustic wave device (SAW), or a micromachined resonator. The high surface area material is preferably an aerogel (or xerogel), but may also include nanotubes, porous carbons, or micromachined materials. The high surface area material is coated with an odorant attracting or detecting material (sensing material), typically a polymer, to which the target molecules attach.
One particular combination of interest is porphyrin coated aerogel. There are a number of known techniques to deposit porphyrins onto substrates, including casting, Langmuir-Blodgett (LB) techniques, and self-assembly deposition. The casting method dissolves an appropriate porphyrin into chloroform and then applies the solution onto the substrate. Once the chloroform evaporates, a thin film of porphyrin will be left behind on the surface of the substrate. However, there is concern for repeatability and uniformity using such an approach to deposit porphyrins into porous structures such as aerogel. The LB techniques are only good for deposition on another layer and not good for filling the pores of a structure. Self-assembly seems appropriate but there are concerns regarding the penetration of the porphyrin into the porous structure of aerogel and furthermore the manufacturing cost and complexity will be high.
Thus it is important to obtain sensors in which the sensing material is strongly attached to the porous substrate material. Otherwise the benefit of the high surface area of the substrate will be lost.